Pregnant women and children have long been warned that they should be wary of eating certain kinds of seafood because of the risk of mercury contamination. It's a real threat — mercury is a neurotoxin, and exposure in-utero at high levels can damage an infant's developing cognitive skills.
Seafood can pose a danger because mercury — usually from the emissions of coal-fired power plants and other industrial sources — can accumulate in the tissue of fish, especially in predators high on the food chain. That includes tuna, and white (albacore) tuna is known to be especially high in mercury. The Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) both recommend that women of childbearing age and young children should eat no more than 12 ounces a week of light tuna, including 6 ounces of white tuna. (More on Time.com: 5 Pregnancy Taboos Explained (or Debunked))
But that may not be safe enough. Consumer Reports tested 42 samples of tuna from cans bought in and around New York and found that white tuna usually contains far more mercury than light tuna — and that women and children should be even more cautious about eating the fish.
After analyzing the tests, the magazine's fish-safety experts concluded that pregnant women should avoid eating all tuna as a precaution. Children over 45 lbs. should stick to no more than 12.5 ounces of light tuna or 4 ounces of white tuna a week, while lighter children should have no more than have 4 ounces or less of light tuna or 1.5 ounces or less of white tuna, dependent on their weight. (Download a copy of the report here.)
Why the stricter warnings? Every sample that Consumer Reports tested had measurable levels of mercury, ranging from 0.018 to 0.774 parts per million (ppm). Samples of white tuna ranged from 0.217 ppm to 0.774 ppm and averaged 0.427 ppm — enough that by eating 2.5 ounces of any of the tested samples, a woman would exceed the daily mercury intake considered safe by the EPA. (More on Time.com: Study: Restless Leg Syndrome During Pregnancy May Recur)
Samples of light tuna ranged from 0.018 ppm to 0.176 ppm. That's low on average, but about half the tested samples contained enough mercury that eating a single can would exceed the EPA's limit for women of child-bearing age.
Indeed, it's the outliers that pose a particular danger, not so much the average. While light tuna especially on average doesn't contain that much mercury, there's the danger of spikes in certain samples — and there's no way for pregnant women to know if the canned tuna they're eating contains unusually high levels of mercury. But the Consumer Reports study shows that it is a real threat that cautious women should take seriously.
Of course, limiting your seafood intake has its own risks. Omega-3 fatty acids — found in fish — are thought to help in developing fetal nervous systems, and they're well-known to reduce the risk of heart attack and stroke. The National Fisheries Institute, a trade group, noted that none of the canned tuna it tested — even the outliers — exceeded the FDA's allowable limit of 1 ppm or more. (That's the point at which the FDA is allowed to pull products from the shelves, though that's never been done.) The group also noted — cheekily — that Consumer Reports had apparently served tuna tartare at its recent holiday party, so it can't be that dangerous. (More on Time.com: Photos: Pregnant Belly Art)
Of course, the FDA's safety limits on mercury have long been considered too lax — and compared to the rest of the world, they are. It will be a long time before we have definitive science on just how much mercury pregnant women can be exposed to without ill effect, but most people would agree that this is a time for the precautionary principle.